Even while SpaceX was perfecting the Falcon 9, they were working on their next rocket, and no one imagined it would be this succesful. The easy way to describe the Falcon Heavy is to say that it's three Falcon 9s strapped together, but that belies the sheer complexity of this vehicle. "It actually ended up being way harder to do Falcon Heavy than we thought," states Elon Musk. "Really way, way more difficult than we originally thought. We were pretty naive about that."